A study of resonance structure in helium using metastable excitation by electron impact with high energy resolution

Abstract

The cross section for excitation of metastable states of helium by electron impact has been studied as a function of incident energy up to the ionization potential. The energy scale is established by determining accurately the position on the metastable spectra of the 2³S and 2¹S levels, whose energies are precisely known from spectroscopic data. The positions of the numerous resonance structures observed in the cross section are determined and the values obtained compared with the results of previous electron transmission and metastable excitation experiments. The energy of the 2²S resonance, observed in an elastic scattering experiment performed simultaneously, is found to be 19.367+or-0.007 eV. A modified Rydberg formula is used to predict energies of the n=3 and 4 resonance states, and the correspondences between the predicted and observed energies are used to suggest classifications for the observed resonances.